We have isolated mutations and deletions in E. coli that define genes required for site-specific recombination. One gene, the himA gene (38.5 minutes on the E. coli linkage map), codes for a protein that is required for temperate phage integration and excision, and is also involved in genetic transposition and possibly transcriptional regulation. We have tentatively identified the product of the himA gene, phimA, as an 8,000 dalton MW protein. We have isolated strains of E. coli (hex) that exhibit high rates of genetic transposition. Analysis of these strains may elucidate the mechanisms of transposition.